Heating resistance flow rate measuring apparatus

ABSTRACT

A heating resistance flow rate measuring apparatus which can use a power source of an ECU in an automobile without requiring an expensive protective circuit and regulator. Heating of temperature detecting resistors themselves causes temperature changes on the windward and leeward sides, and these temperature changes depend on the amounts of heat generated by the temperature detecting resistors. The amounts of heat generated by the temperature detecting resistors depend on a voltage value of the ECU power source applied to the temperature detecting resistors, and an error is caused in a sensor output depending on a variation in the voltage of the ECU power source. Based on the finding that suppressing the amount of heat generated by the temperature detecting resistors is effective in avoiding the sensor output error caused depending on a variation in the output voltage of the ECU power source, means for suppressing the amount of heat generated by the temperature detecting resistors is disposed.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Continuation application of U.S. application Ser.No. 11/052,754, filed Feb. 9, 2005, which is incorporated by referenceherein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a heating resistance flow ratemeasuring apparatus, and more particularly to a heating resistance flowrate measuring apparatus for measuring the flow rate of a fluid based ona detected temperature difference between temperature detectingresistors disposed on both sides of a heating resistor.

2. Description of the Related Art

One type of known heating resistance flow rate measuring apparatuscomprises a heating resistor and temperature detecting resistorsdisposed respectively upstream and downstream of the heating resistor inthe flowing direction of a fluid. The flow rate of the fluid isdetermined by measuring a temperature difference between the temperaturedetecting resistors on the upstream and downstream sides.

As one example of that type of heating resistance flow rate measuringapparatus, there is a thermal type airflow sensor disclosed in PatentReference 1; JP,A 2002-48616.

SUMMARY OF THE INVENTION

In the known heating resistance flow rate measuring apparatus comprisingthe heating resistor and the temperature detecting resistors disposed onboth the sides of the heating resistor, a battery is employed as a powersource, for example, when the flow rate measuring apparatus is mountedin an automobile.

On the other hand, a pressure sensor and the like used in an automobileare generally supplied with ECU power source, i.e., power source from anECU (Engine Control Unit), instead of power from the battery.

The reason why the pressure sensor and the like are supplied with thepower source from the ECU resides in that, because the voltage of theECU power source is about 5 V, requirements for specifications ofelectric noise, overvoltage, etc. can be set to lower voltage levels anda protective circuit necessary for meeting the requirements can beformed by using parts that are endurable against relatively lowvoltages, thus resulting in reduction of the cost.

In other words, when the power is supplied from the battery instead ofthe ECU power source, requirements for specifications of electric noise,overvoltage, etc. are severer and parts endurable against relativelyhigh voltages are required for protection of the pressure sensor and thelike.

For the heating resistance flow rate measuring apparatus, the power issupplied from the battery instead of the ECU power source because thepower consumption of the flow rate measuring apparatus is large.

Modifying the heating resistance flow rate measuring apparatus to becapable of being supplied with the power source from the ECU instead ofthe battery seems possible just by reducing the power consumption. Whenreceiving the power source supplied from the ECU, however, the voltageof the power source is required to be held in the range of 4.5 to 5.5 Vfor operation of the flow rate measuring apparatus, and the currentthereof is also limited to ten and several mA. Further, the outputvoltage of the flow rate measuring apparatus must be in proportion tothe voltage value of the ECU power source.

For that reason, an expensive protective circuit and regulator arerequired, and the overall cost of the device is pushed up.

Accordingly, it is an object of the present invention to realize aheating resistance flow rate measuring apparatus which can use a powersource of an ECU in an automobile without requiring an expensiveprotective circuit and regulator.

To achieve the above object, the present invention is constructed asfollows.

In a heating resistance flow rate measuring apparatus comprising aheating resistor and temperature detecting resistors disposedrespectively upstream and downstream of the heating resistor in theflowing direction of a fluid, a heating suppressing unit for suppressingheating of the temperature detecting resistors is disposed.

When the temperature detecting resistors are directly connected to powersource, the amounts of heat generated by the temperature detectingresistors are changed depending on a change in voltage value of thepower source, and the temperatures of the temperature detectingresistors themselves are also changed. Accordingly, an output voltage ofthe heating resistance flow rate measuring apparatus becomes notproportional to the voltage value of the power source.

Based on that finding, the heating suppressing unit for suppressingheating of the temperature detecting resistors is disposed to make theoutput voltage of the heating resistance flow rate measuring apparatussubstantially proportional to the voltage value of the power source.

One preferable example of the heating suppressing unit for thetemperature detecting resistors is to set a resistance value of thetemperature detecting resistors to a high value in the range of 5 kΩ to500 kΩ. Another preferable example of the heating suppressing unit is aPeltier device serving as a device for cooling the temperature detectingresistor. Still another preferable example of the heating suppressingunit is a unit for intermittently supplying a current to the temperaturedetecting resistors.

Further, the larger the resistance temperature coefficient of theheating resistor, the larger is a variation in resistance value of theheating resistor. On the other hand, if the resistance temperaturecoefficient of the heating resistor were too small, it would bedifficult to detect the temperature of the heating resistor. Inaddition, the temperature of the heating resistor is required to beincreased to a level not lower than a certain temperature with the viewof ensuring the function of removing contaminants. For those reasons,the resistance temperature coefficient of the heating resistor ispreferably in the range of 1000 ppm/° C. to 2000 ppm/° C.

Thus, according to the present invention, it is possible to realize aheating resistance flow rate measuring apparatus which can use a powersource of an ECU in an automobile without requiring an expensiveprotective circuit and regulator.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a sensor element of a heating resistance flowrate measuring apparatus according to a first embodiment of the presentinvention;

FIG. 2 is a sectional view taken along the line A-A′ in FIG. 1;

FIG. 3 is a diagram of a driving circuit of the heating resistance flowrate measuring apparatus according to the first embodiment;

FIG. 4 is a graph showing the relationship between a voltage value ofECU power source and a sensor output;

FIG. 5 is a graph showing the relationship between a ratio of the amountof heat generated by temperature detecting resistors to the amount ofheat generated by a heating resistor and a ratio error;

FIG. 6 is a graph showing the relationship between a resistance value ofthe temperature detecting resistors and the ratio error;

FIG. 7 is a graph showing the relationship of a resistance value of theheating resistor versus a current and a voltage which are required forcausing the heating resistor to generate heat of 45 mW;

FIG. 8 is a graph showing the relationship of a resistance value of theheating resistor versus a current and a voltage which are required forcausing the heating resistor to generate heat of 40 mW;

FIG. 9 is a graph showing the relationship between the amount of heatgenerated by the heating resistor and a resistance variation allowablevalue of the heating resistor;

FIG. 10 is a graph showing the relationship between the resistancetemperature coefficient of the heating resistor and the amount of heatgenerated by the heating resistor;

FIG. 11 is an enlarged view of the heating resistor and the temperaturedetecting resistors;

FIG. 12 is a circuit diagram for measuring the resistance value of theheating resistor and the resistance value of the temperature detectingresistors;

FIG. 13 is a diagram of a driving circuit of a heating resistance flowrate measuring apparatus according to a second embodiment of the presentinvention;

FIG. 14 is a diagram of a driving circuit of a heating resistance flowrate measuring apparatus according to a third embodiment of the presentinvention;

FIG. 15 is a diagram of a driving circuit of a heating resistance flowrate measuring apparatus according to a fourth embodiment of the presentinvention;

FIG. 16 is a block diagram of principal components of an operationcontrol unit in which the heating resistance flow rate measuringapparatus according to any of the embodiments of the present inventionis employed; and

FIG. 17 is a diagram of a driving circuit of a known heating resistanceflow rate measuring apparatus, which is supplied with power from abattery unlike the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will be described below withreference to the drawings.

First, a heating resistance flow rate measuring apparatus according to afirst embodiment of the present invention will be described withreference to FIGS. 1 to 12. Of these drawings, FIG. 1 is a plan view ofa sensor element 1 of the heating resistance flow rate measuringapparatus according to the first embodiment of the present invention,and FIG. 2 is a sectional view taken along the line A-A′ in FIG. 1. FIG.3 is a diagram of a driving circuit of the heating resistance flow ratemeasuring apparatus according to the first embodiment, and FIG. 4 is agraph showing the relationship between a voltage value of ECU powersource and a sensor output.

FIG. 5 is a graph showing the relationship between a ratio of the amountof heat generated by temperature detecting resistors 3, 4, 7 and 8 tothe amount of heat generated by a heating resistor 6 and a ratio error,FIG. 6 is a graph showing the relationship between a resistance value ofthe temperature detecting resistors 3, 4, 7 and 8 and the ratio error,and FIG. 7 is a graph showing the relationship of a resistance value ofthe heating resistor 6 versus a current and a voltage which are requiredfor causing the heating resistor 6 to generate heat of 45 mW.

FIG. 8 is a graph showing the relationship of a resistance value of theheating resistor 6 versus a current and a voltage which are required forcausing the heating resistor 6 to generate heat of 40 mW, and FIG. 9 isa graph showing the relationship between the amount of heat generated bythe heating resistor 6 and a resistance variation allowable value of theheating resistor 6. FIG. 10 is a graph showing the relationship betweenthe resistance temperature coefficient of the heating resistor 6 and theamount of heat generated by the heating resistor 6, FIG. 11 is anenlarged view of the heating resistor 6 and the temperature detectingresistors 3, 4, 7 and 8, and FIG. 12 is a circuit diagram for measuringthe resistance value of the heating resistor 6 and the resistance valueof the temperature detecting resistors 3, 4, 7 and 8. Additionally, forcomparison with the present invention, FIG. 17 shows a driving circuitof a known heating resistance flow rate measuring apparatus, which issupplied with power from a battery unlike the present invention.

The construction of the sensor element 1 of the heating resistance flowrate measuring apparatus according to the first embodiment of thepresent invention will be described with reference to FIGS. 1 and 2.

In FIGS. 1 and 2, a flat substrate 23 of the sensor element 1 is made ofa material having a high thermal conductivity, such as silicon andceramic. After forming an insulating film 22 on the flat substrate 23,the flat substrate 23 is etched away from its rear side to form a cavitybelow the insulating film 22. A thin wall portion (diaphragm) 2 isthereby formed in the flat substrate 23.

On the surface of the thin wall portion 2, there are formed a heatingresistor 6 which is heated so as to hold a certain temperaturedifference with respect to the temperature of airflow to be measured,and temperature detecting resistors 3, 4, 7 and 8 disposed on both sidesof the heating resistor 6. The heating resistor 6 is a resistor made of,for example, a polysilicon thin film, a platinum thin film, or a nickelalloy thin film. The heating resistor 6 generates heat when a current issupplied to flow through it, and has a resistance value changeddepending on the temperature thereof.

Similarly, the temperature detecting resistors 3, 4, 7 and 8 are each aresistor made of, for example, a polysilicon thin film, a platinum thinfilm, or a nickel alloy thin film, and has a resistance value changeddepending on the temperature thereof.

Thus, in the heating resistance flow rate measuring apparatus having theabove-described arrangement, the heating resistor 6 is heated such thata certain temperature difference is held with respect to the temperatureof airflow to be measured. When air flows through the heating resistanceflow rate measuring apparatus, the temperature on the windward(upstream) side of the heating resistor 6 lowers, while the temperatureon the leeward (downstream) side of the heating resistor 6 rises. Inconsideration of such a phenomenon, the flow rate of the air is measuredby detecting a resulting temperature change with the temperaturedetecting resistors 3, 4, 7 and 8.

Additionally, based on the fact that the temperature of the flatsubstrate 23 is changed depending on the ambient temperature, theambient temperature is detected by a temperature measuring resistor 10which is also disposed on the flat substrate 23.

Further, by detecting the ambient temperature in accordance with theinformation obtained from the temperature measuring resistor 10, thetemperature of the heating resistor 6 is controlled so that a certaintemperature difference is held with respect to the ambient temperature.The temperature measuring resistor 10 is a resistor made of, forexample, a polysilicon thin film, a platinum thin film, or a nickelalloy thin film, and serves to measure the ambient temperature byutilizing the fact that its resistance value is changed depending on thetemperature thereof.

The heating resistor 6, the temperature detecting resistors 3, 4, 7 and8, and the temperature measuring resistor 10 are connected to pads 11,12, 13, 14, 15, 16, 17, 18, 19, 20 and 21 for allowing the wiring to beled out to the exterior.

The heating resistor 6 and the temperature measuring resistor 10 areconnected in series through a wiring pattern 9 formed on the flatsubstrate 23, and a junction between the heating resistor 6 and thetemperature measuring resistor 10 is connected to the pad 17 through thewiring pattern 9. Moreover, the heating resistor 6 is connected to thepad 15 through a wiring pattern 5.

Next, the arrangement of a driving circuit of a known heating resistanceflow rate measuring apparatus, which is supplied with power from abattery unlike the present invention, will be described with referenceto FIG. 17 for the purpose of comparison.

As shown in FIG. 17, a driving circuit 24 receiving power source from abattery comprises a power source terminal 26 to which the power sourceis supplied from the battery, and a protective circuit 25 for protectingthe driving circuit 24 against electric noise and overvoltage of thepower source supplied from the battery. The driving circuit 24 furthercomprises a heating resistor 6 and a temperature measuring resistor 10both disposed within a sensor element similar to that denoted by 1 inFIG. 1, a driving transistor 27 for energizing the heating resistor 6,resistances 30, 31 connected in parallel to a circuit made up of theheating resistor 6 and the temperature measuring resistor 10 connectedin series, and an amplifier 29 for amplifying an error voltage of abridge circuit made of the heating resistor 6, the temperature measuringresistor 10, and the resistances 30, 31 and energizing the drivingtransistor 27.

Still further, the driving circuit 24 comprises a regulator 28 forgenerating a constant voltage from an output of the protective circuit25 and supplying the constant voltage to temperature detecting resistors3, 4, 7 and 8, an amplifier 32 for amplifying an output voltage of abridge circuit made up of the temperature detecting resistors 3, 4, 7and 8 to generate a sensor output, and a sensor output terminal 33 foroutputting the sensor output to the exterior.

In the driving circuit 24 thus arranged, because the power source issupplied from the battery, there are substantially no restrictions onthe power, and significant problems are not caused even if the drivingcircuit 24 consumes a current on the order of several amperes.Furthermore, with the provision of the regulator 28 in the drivingcircuit 24, variations hardly occur in the voltage applied to thetemperature detecting resistors 3, 4, 7 and 8.

Next, the arrangement of a driving circuit of the heating resistanceflow rate measuring apparatus according to the first embodiment of thepresent invention will be described with reference to FIG. 3.

As shown in FIG. 3, a driving circuit 34 according to the firstembodiment of the present invention comprises a power source terminal 35to which ECU power source is supplied from an ECU 42, the heatingresistor 6 and the temperature measuring resistor 10 both disposedwithin the sensor element 1, a driving transistor 36 for energizing theheating resistor 6, resistances 38, 39 connected in parallel to acircuit made up of the heating resistor 6 and the temperature measuringresistor 10 connected in series, and an amplifier 37 for amplifying anerror voltage of a bridge circuit made of the heating resistor 6, thetemperature measuring resistor 10, and the resistances 38, 39 andenergizing the driving transistor 36.

The driving circuit 34 further comprises an amplifier 40 for amplifyingan output voltage of a bridge circuit made up of the temperaturedetecting resistors 3, 4, 7 and 8 to generate a sensor output, and asensor output terminal 41 for outputting the sensor output to theexterior.

In the driving circuit 34 thus arranged, because the power source issupplied as the ECU power source from the ECU 42, there are variousrestrictions on the power.

First, the voltage value is about 5 V, the minimum operation assurancevoltage is required to be 4.5 V, and the current is limited to about 10mA. Further, because the voltage value of the ECU power source variesfor each ECU used in practice, the sensor output is required to be inproportion to the voltage value of the ECU power source as indicated bya reference line in FIG. 4.

On the other hand, since the voltage value of the ECU power source is aslow as about 5 V, neither the protective circuit 25 adapted for arelatively high-voltage level, nor the regulator 28 are necessary unlikethe case of receiving the power from the battery as shown in FIG. 17.Therefore, the use of the ECU power source is very effective in reducingthe cost of the heating resistance flow rate measuring apparatus.

For that reason, as described above, it has become more popular toutilize the ECU power source supplied from the ECU for a pressure sensorand a temperature sensor used in an automobile. However, because thecurrent consumption of the heating resistance flow rate measuringapparatus is large, there has been a difficulty in utilizing the ECUpower source for the flow rate measuring apparatus.

In particular, a hot-wire flow rate measuring apparatus has a difficultyin reducing power consumption of a heating resistor, and the ECU powersource has been kept from being employed for the hot-wire flow ratemeasuring apparatus.

Meanwhile, in a heating resistance flow rate measuring apparatus of thetype that a diaphragm is formed in a semiconductor substrate, e.g., asilicon substrate, and a heating resistor is formed on the diaphragm,like the above-described heating resistance flow rate measuringapparatus disclosed in Patent Reference 1, the size of the heatingresistor can be reduced.

The size reduction of the heating resistor contributes to reducing heatcapacity of the heating resistor and hence to realizing smaller powerconsumption. From this point of view, it seems possible to employ theECU power source, which supplies lower power than the battery, in theheating resistance flow rate measuring apparatus as well.

However, other various problems in addition to the power consumptionmust be overcome to realize practical use of the ECU power source in theheating resistance flow rate measuring apparatus. Those problems andcountermeasures will be described below.

When the ECU power source is employed for supply of the power to theheating resistance flow rate measuring apparatus, the flow ratemeasuring apparatus is required to have a characteristic, as shown inFIG. 4, that the sensor output is in proportion to the voltage value ofthe ECU power source (such a characteristic is defined here as aratiometric characteristic).

In the driving circuit 34 shown in FIG. 3, therefore, the ECU powersource is connected to supply the power for the bridge circuit made upof the temperature detecting resistors 3, 4, 7 and 8 so that the outputvoltage of the bridge circuit made up of the temperature detectingresistors 3, 4, 7 and 8 is in proportion to the voltage value of the ECUpower source.

In the actual circuit, however, the sensor output is not in proportionto the voltage value of the ECU power source as indicated by a thicksolid line r in FIG. 4, and a voltage error generates relative to theoutput voltage (indicated by the reference line) that is in proportionto the voltage value of the ECU power source. Such a voltage error isdefined here as a ratio error. As a result of experiments made by theinventors, it has been found that, as shown in FIG. 5, the ratio erroris in proportion to a ratio of the amount of heat generated by thetemperature detecting resistors 3, 4, 7 and 8 to the amount of heatgenerated by the heating resistor 6.

Further, the inventors have confirmed that the above-mentioned result isattributable to the fact that, since the temperature detecting resistors3, 4, 7 and 8 disposed upstream and downstream of the heating resistor 6are directly connected to the ECU power source, the amount of heatgenerated by temperature detecting resistors 3, 4, 7 and 8 is changeddepending on a change in the voltage value of the ECU power source, andthe temperatures of the temperature detecting resistors 3, 4, 7 and 8themselves are also changed correspondingly.

In the heating resistance flow rate measuring apparatus, as describedabove, the heating resistor 6 is heated such that a certain temperaturedifference is held with respect to the ambient temperature. When thereoccurs a wind (flow of air as a fluid) in the heating resistance flowrate measuring apparatus, the temperature on the windward (upstream)side of the heating resistor 6 lowers, while the temperature on theleeward (downstream) side of the heating resistor 6 rises. Inconsideration of such a phenomenon, the above temperature change ismeasured by detecting resistance changes of the temperature detectingresistors 3, 4, 7 and 8.

The heating of the temperature detecting resistors 3, 4, 7 and 8themselves causes temperature changes on the windward and leeward sides,and these temperature changes depend on the amounts of heat generated bythe temperature detecting resistors 3, 4, 7 and 8. Then, the amounts ofheat generated by the temperature detecting resistors 3, 4, 7 and 8depend on the voltage applied to the temperature detecting resistors 3,4, 7 and 8, i.e., the voltage value of the ECU power source.

Thus, it is deemed that, corresponding to a variation in the voltagevalue of the ECU power source, an error occurs in the sensor output,whereby the ratio error is generated.

To prevent the sensor output error caused by a variation in the voltagevalue of the power source, the known heating resistance flow ratemeasuring apparatus employs the regulator 28 as shown in FIG. 17.

In this embodiment of the present invention in which the power source issupplied from the ECU, however, the ECU power source must be directlyconnected to the bridge circuit made up of the temperature detectingresistors 3, 4, 7 and 8 in order to obtain the ratiometriccharacteristic.

Taking into account the above, in the present invention, the amount ofheat generated by the temperature detecting resistors 3, 4, 7 and 8 issuppressed to avoid the sensor output error caused by a variation in thevoltage value of the ECU power source in the arrangement where the ECUpower source is directly connected to the bridge circuit, withoutinviting a substantial increase in cost.

A practically conceivable means for suppressing the amount of heatgenerated by the temperature detecting resistors 3, 4, 7 and 8 is toreduce the amount of heat generated by the temperature detectingresistors 3, 4, 7 and 8 in itself, or to cool the temperature detectingresistors 3, 4, 7 and 8.

One example of the means for reducing the amount of heat generated bythe temperature detecting resistors 3, 4, 7 and 8 in itself is toincrease the resistance value of the temperature detecting resistors 3,4, 7 and 8.

The example of increasing the resistance value of the temperaturedetecting resistors 3, 4, 7 and 8 will be described below.

FIG. 6 is a graph showing the result of experiments made on therelationship between the resistance value of the temperature detectingresistors 3, 4, 7 and 8 and the ratio error.

As seen from FIG. 6, the ratio error can be held within the allowablerange by setting the resistance value of the temperature detectingresistors 3, 4, 7 and 8 to be not smaller than 5 kΩ. This is presumablyresulted from the fact that the increased resistance value of thetemperature detecting resistors 3, 4, 7 and 8 reduces the amount of heatgenerated by the temperature detecting resistors 3, 4, 7 and 8 and hencecontributes to realizing a reduction of the ratio error.

As shown in FIG. 5, the ratio error depends on the ratio of the amountof heat generated by the temperature detecting resistors 3, 4, 7 and 8to the amount of heat generated by the heating resistor 6. Then, whenthe driving circuit 34 is supplied with the power source from the ECU asin the present invention, the amount of heat generated by the heatingresistor 6 is limited to about 40 mW because of restrictions on thevoltage and current of the ECU power source. Further, the voltage of theECU power source is fixed to about 5 V, and therefore the amount of heatgenerated by the temperature detecting resistors 3, 4, 7 and 8 dependson the resistance value of the temperature detecting resistors 3, 4, 7and 8.

From the restrictive conditions described above, in the case using theECU power source, the resistance value of the temperature detectingresistors 3, 4, 7 and 8 is essentially required to be not smaller than 5kΩ. A preferable upper limit is 500 kΩor more in consideration of theinfluence of input impedance of the amplifier 40. In practical use, asufficient effect is expected with the resistance value in the range of5 kΩ to 500 kΩ because the ratio error can be held within the allowablerange by using the resistance value in that range.

Also, corresponding to the limitation on the amount of heat generated bythe heating resistor 6, the heating resistor 6 is required to have asmaller area. When the amount of heat generated by the heating resistor6 is set to 40 mW, for example, the area of the heating resistor 6 mustbe 0.1 mm² or less.

In other words, the heating resistor 6 must be sized such that thelength is not more than 500 μm and the width is not more than 200 μm. Asthe area of the heating resistor 6 reduces, the area in which thetemperature detecting resistors 3, 4, 7 and 8 are to be disposed is alsoreduced (namely, a region where temperature is changed with airflow is avery limited one around the heating resistor 6, and the temperaturedetecting resistors 3, 4, 7 and 8 must be disposed in such a region).For that reason, if the temperature detecting resistors 3, 4, 7 and 8are formed in the same film thickness and line width as those in theknown art, the resistance value of the temperature detecting resistors3, 4, 7 and 8 becomes small.

In this embodiment of the present invention, therefore, for the purposeof obtaining the resistance value of the temperature detecting resistors3, 4, 7 and 8 not smaller than 5 kΩ, the temperature detecting resistors3, 4, 7 and 8 are each formed of a thin film having a film thickness ofnot more than 0.1 μm and a line width of not more than 1 μm when thoseresistors are made of platinum or a nickel alloy. Also, when using apolysilicon thin film, the temperature detecting resistors 3, 4, 7 and 8are each formed of a thin film having a film thickness of not more than0.5 μm and a line width of not more than 4 μm. From the viewpoints ofprocess efficiency and variations, the polysilicon thin film is moreadvantageous in practical use.

Thus, according to the first embodiment of the present invention, sincethe resistance value of the temperature detecting resistors 3, 4, 7 and8 is set to a large value (5 kΩ to 500 kΩ) to suppress the amount ofheat generated by the temperature detecting resistors 3, 4, 7 and 8, theoutput value is prevented from varying to a large extent depending onthe power source voltage, and a stable output value can be obtained evenin spite of using the ECU power source as a power source for the flowrate measuring apparatus.

Consequently, a heating resistance flow rate measuring apparatus capableof using, as a power source, the ECU power source produced in anautomobile can be realized without requiring an expensive protectivecircuit and regulator.

Further, the inventors have clarified that, when the ECU power source isemployed as the power source for the heating resistance flow ratemeasuring apparatus, the resistance temperature coefficient of theheating resistor is a very important factor. This point will bedescribed below.

Hitherto, it has been commonsense in the art that the resistancetemperature coefficient of the heating resistor 6 is preferably as largeas possible from the viewpoint of measuring the temperature of theheating resistor 6 based on a change in the resistance value of theheating resistor 6. For that reason, a material having a largeresistance temperature coefficient of 4000 ppm/° C., such as platinum ora nickel alloy, has been used to form the heating resistor 6.

According to the studies made by the inventors, however, it has beenfound that, when the driving circuit 34 receives the power sourcesupplied from the ECU, the resistance temperature coefficient of theheating resistor 6 should be not more than about 2000 ppm/° C.

FIG. 7 is a graph showing the relationship of the resistance value ofthe heating resistor 6 versus the current and the voltage which arerequired for causing the heating resistor 6 to generate heat of 45 mW.Assuming here that the current of the power source supplied from the ECUis limited 10 mA and the voltage thereof is limited to 5 V, theallowable variation range for the resistance value of the heatingresistor 6 is 450 Ω to 550 Ω.

In the case of the resistance temperature coefficient being as high as4000 ppm/° C. like platinum or a nickel alloy, however, when the ambienttemperature changes 100° C., the resistance value of the heatingresistor 6 is changed 40% with such a change in the ambient temperaturealone.

Supposing that the resistance value of the heating resistor 6 is 500 Ωat 25° C., the resistance value of the heating resistor 6 at 125° C. is700 Ω. In this case, because the voltage is limited to 5 V, the powercapable of being supplied to the heating resistor 6 is just 36 mW,whereby the temperature of the heating resistor 6 lowers and thecharacteristics of the heating resistance flow rate measuring apparatusare significantly changed.

To enlarge the allowable range of the resistance variation, therefore,it is required to form the heating resistor 6 in smaller size and toreduce the power required for the heating resistor 6.

FIG. 8 is a graph showing the relationship of the resistance value ofthe heating resistor 6 versus the current and the voltage which arerequired for causing the heating resistor 6 to generate heat of 40 mW.As seen from FIG. 8, by reducing the amount of heat generated by theheating resistor 6 to 40 mW, the allowable range of the resistancevariation spans from 400 Ω to 620 Ω; namely the resistance variationallowable range of the heating resistor 6 is enlarged.

That relationship is plotted in FIG. 9. Specifically, FIG. 9 is a graphshowing the relationship between the amount of heat generated by theheating resistor 6 and a resistance variation allowable value (%) of theheating resistor 6. As seen from FIG. 9, too, the resistance variationallowable value of the heating resistor 6 can be increased by reducingthe amount of heat generated by the heating resistor 6.

However, it is generally known that reducing the amount of heatgenerated by the heating resistor 6 makes the heating resistor 6 moresusceptible to contamination. This is because even when water or oil isattached to the heating resistor 6, the attached water or oil can beevaporated if the amount of heat generated by the heating resistor 6 islarge. Stated another way, if the amount of heat generated by theheating resistor 6 is small, the attached water or oil cannot beevaporated and the heating resistor 6 is more susceptible tocontamination.

From that reason, the amount of heat generated by the heating resistor 6is preferably as large as possible from the viewpoint of protectionagainst contamination.

FIG. 10 is a graph showing the relationship between the resistancetemperature coefficient of the heating resistor 6 and the amount of heatgenerated by the heating resistor 6 in consideration of a resistancevariation width of the heating resistor 6 resulting from a change in theresistance temperature of the heating resistor 6. As is apparent fromFIG. 10, as the resistance temperature coefficient of the heatingresistor 6 increases, the resistance variation of the heating resistor 6is increased and the amount of heat generated by the heating resistor 6is reduced.

Then, in order to suppress the amount of heat generated by the heatingresistor 6 within the allowable range, the resistance temperaturecoefficient of the heating resistor 6 is required to be at least 2000ppm/° C. or less.

In the first embodiment of the present invention, therefore, thecomposition of a platinum or nickel alloy, i.e., a material of theheating resistor 6, is modified so as to reduce the resistancetemperature coefficient of the heating resistor 6. Also, when apolysilicon this film is used as the material of the heating resistor 6,the thin film is subjected to doping at such a high density that theresistance temperature coefficient of 2000 ppm/° C. is obtained.

On the other hand, if the resistance temperature coefficient of theheating resistor 6 were too small, it would be difficult to detect thetemperature in a reliable manner. Therefore, the resistance temperaturecoefficient is required to be at least 1000 ppm/° C. or more.

The construction of the heating resistor 6 and the temperature detectingresistors 3, 4, 7 and 8 will be described below with reference to FIG.11.

In FIG. 11, the heating resistor 6 is formed in small area so that itcan be heated to high temperature with a relatively small amount of heatgenerated. Practically, the area of the heating resistor 6 is requiredto be not larger than 0.1 mm². In this embodiment of the presentinvention, the heating resistor 6 is designed in area of 500 μm×200 μm.

Also, slits 43, 44, 45, 46, 47 and 48 are formed in the heating resistor6 to provide a resistance value at which maximum efficiency is achievedwhen the driving circuit 34 receives the power source supplied from theECU. Stated another way, the heating resistor 6 is made up of aplurality of strip-shaped resistors for optimum adjustment of theresistance value thereof.

Further, forming the heating resistor 6 to be made up of a plurality ofstrip-shaped resistors contributes to eliminating unevenness intemperature of the heating resistor 6. If the heating resistor 6 were inthe form of a single strip, the temperature of the heating resistor 6 isdistributed such that the temperature in a central portion is relativelyhigh and the temperature in a peripheral portion is relatively low.

With the heating resistor 6 being made up of a plurality of strip-shapedresistors as shown in FIG. 11, when the temperature in the centralportion of the heating resistor 6 is going to rise, the resistance valueof the strip-shaped resistor in the central portion is increased so asto reduce the amount of heat generated. On the other hand, when thetemperature in the peripheral portion of the heating resistor 6 is goingto lower, the resistance value of the strip-shaped resistor in theperipheral portion is reduced so as to increase the amount of heatgenerated. As a result, the temperature of the heating resistor 6 can bekept uniform.

The resistance value of the heating resistor 6 at which maximumefficiency is achieved when the driving circuit 34 receives the powersource supplied from the ECU is given as a value resulting from dividingthe minimum operation assurance voltage of the power source supplied tothe heating resistance flow rate measuring apparatus by the maximumallowable current of the power source supplied to the heating resistanceflow rate measuring apparatus. When the heating resistance flow ratemeasuring apparatus is supplied with the power source from the ECU, theaforesaid resistance value is 450 Ω on condition that the minimumoperation assurance voltage is 4.5 V and the maximum allowable currentis 10 mA.

Accordingly, a total of the resistance value of the heating resistor 6and the resistance value of wired lines for the heating resistor 6 ispreferably set to be about 450 Ω±20%, taking into account processvariations. Note that the above total resistance value represents avalue when the heating resistor 6 is in a heated state.

While the heating resistor 6 and the temperature detecting resistors 3,4, 7 and 8 are set so as to have the resistance temperature coefficientand the resistance values described above, it is preferable to confirmwhether the resistance temperature coefficient and the resistance valuessatisfy the respective setting values or not after those resistors havebeen actually formed in the driving circuit 34.

Methods for measuring the resistance temperature coefficient of theheating resistor 6 and the resistance value of the temperature detectingresistors 3, 4, 7 and 8 will be described below with reference to FIG.12.

FIG. 12 is a circuit diagram of the heating resistance flow ratemeasuring apparatus. As shown, the heating resistor 6 and thetemperature measuring resistor 10 are connected in series, and thewiring including these resistors 6, 10 is led out to the exteriorthrough the pads 15, 16 and 17. Also, the wiring including thetemperature detecting resistors 3, 4, 7 and 8 is led out to the exteriorthrough the pads 11, 12, 13, 14, 18, 19, 20 and 21.

The method of measuring the resistance temperature coefficient of theheating resistor 6 is first described.

A measuring circuit is formed by connecting a voltage source 50 and anammeter 49, which are connected in series, between the pad 15 connectedto one end of the heating resistor 6 and the pad 17 connected to theother end of the heating resistor 6.

Then, a current is supplied from the voltage source 50 to the heatingresistor 6 for heating the heating resistor 6 to a predeterminedtemperature, and a value of the ammeter 49 at each of the ambienttemperatures of 25° C. and 85° C. is detected.

As a result, a total resistance value of the heating resistor 6 andwired leads for the heating resistor 6 at each of the ambienttemperatures of 25° C. and 85° C. is measured. From this measured totalresistance value, the resistance temperature coefficient of the heatingresistor 6 is calculated.

The method of measuring the resistance value of the temperaturedetecting resistors 3, 4, 7 and 8 is next described.

A measuring circuit is formed by connecting the pad 11 connected to oneend of the resistor 3 and the pad 18 connected to one end of theresistor 7 to each other, connecting the pad 14 connected to the otherend of the resistor 3 and the pad 19 connected to one end of theresistor 8 to each other, connecting the pad 21 connected to the otherend of the resistor 7 and the pad 12 connected to one end of theresistor 4 to each other, and connecting the pad 20 connected to theother end of the resistor 8 and the pad 13 connected to the other end ofthe resistor 4 to each other. A bridge circuit made up of thetemperature detecting resistors 3, 4, 7 and 8 is thereby constructed.

A voltage source 52 and an ammeter 51, which are connected in series,are connected between the junction of the pads 11, 18 and the junctionof the pads 13, 20. The voltage source 52 is set to 5 V, and a value ofthe ammeter 51 at the ambient temperature of 25° C. is read. Then, theresistance value of the temperature detecting resistors 3, 4, 7 and 8 iscalculated from the value of the ammeter 51.

The arrangement of a driving circuit of a heating resistance flow ratemeasuring apparatus according to a second embodiment of the presentinvention will be described below with reference to FIG. 13.

As shown in FIG. 13, the driving circuit in the second embodimentreceives the power source supplied from the ECU, and it comprises theheating resistor 6 and the temperature measuring resistor 10 bothdisposed within the sensor element 1, a driving transistor 55 forenergizing the heating resistor 6, resistances 56, 57 connected inparallel to a circuit made up of the heating resistor 6 and thetemperature measuring resistor 10 connected in series, an amplifier 53for amplifying an error voltage of a bridge circuit made of the heatingresistor 6, the temperature measuring resistor 10, and the resistances56, 57, and a PWM circuit 54 for generating a pulse modulation signalfrom an output of the amplifier 53 and energizing the driving transistor55.

The driving circuit in the second embodiment further comprises anamplifier 58 for amplifying an output voltage of a bridge circuit madeup of the temperature detecting resistors 3, 4, 7 and 8 to generate asensor output.

The driving circuit in the second embodiment has an arrangement commonto the driving circuit in the first embodiment, and the resistance valueof the temperature detecting resistors 3, 4, 7 and 8 is in the range of5 kΩ to 500 kΩ. The difference between the first and second embodimentsresides in a method for energizing the heating resistor 6.

More specifically, in the second embodiment of the present invention,the temperature of the heating resistor 6 is controlled by performingon/off control of the driving transistor 55 in accordance with the pulsemodulation signal from the PWM circuit 54.

In the driving circuit shown in FIG. 13, therefore, the resistance valueof the temperature measuring resistor 10 is set larger than that of theheating resistor 6 so that there occurs a difference between the amountof heat generated by the heating resistor 6 and the amount of heatgenerated by the temperature measuring resistor 10 with the on/offcontrol of the driving transistor 55.

The second embodiment can also provide similar advantages to thoseobtained with the first embodiment.

A third embodiment of the present invention will be described below withreference to FIG. 14.

In the above-described first embodiment of the present invention, theresistance value of the temperature detecting resistors 3, 4, 7 and 8 isset to a high value (not less than 5 kΩ) for the purpose of suppressingthe amount of heat generated by the temperature detecting resistors 3,4, 7 and 8. On the other hand, the resistance value of temperaturedetecting resistors 70, 71, 72 and 73 in this third embodiment is set toa low value of less than 5 kΩ instead of a high value, and Peltierdevices 74, 75, 76 and 77 are disposed respectively near the temperaturedetecting resistors 70 to 73 as means for suppressing temperature riseof the resistors 70 to 73. The Peltier devices 74, 75, 76 and 77 aredriven by a Peltier device driving circuit 78 so that the temperaturedetecting resistors 70 to 73 are held at appropriate temperatures.

The remaining arrangement of the third embodiment is the same as that ofthe first embodiment.

Since the Peltier devices and the driving circuit for the Peltierdevices are inexpensive as compared with the protective circuit 25 andthe regulator 28 shown in FIG. 17, the third embodiment of the presentinvention can also provide similar advantages to those obtained with thefirst embodiment.

A fourth embodiment of the present invention will be described belowwith reference to FIG. 15.

In this fourth embodiment, as in the third embodiment, the resistancevalue of the temperature detecting resistors 70, 71, 72 and 73 is set toa low value of less than 5 kΩ instead of a high value, and a circuit forcontrolling an applied current value is disposed as means forsuppressing temperature rise of the resistors 70 to 73. Morespecifically, a driving circuit 79 capable of being subjected to on/offcontrol, such as a transistor, is connected to the bridge circuit madeup of the temperature detecting resistors 70 to 73, and energization ofthe driving circuit 79 is controlled in accordance with a pulse signalfrom a pulse generator 80.

Then, an output signal of the amplifier 40 is supplied to a sensoroutput terminal 41 through a sample hold circuit 81. The sample holdcircuit 81 sample-holds and outputs the signal supplied to it inaccordance with the pulse signal from a pulse generator 80.

The remaining arrangement of the fourth embodiment is the same as thatof the first embodiment.

Since the driving circuit 79, the pulse generator 80, and the samplehold circuit 81 are inexpensive as compared with the protective circuit25 and the regulator 28 shown in FIG. 17, the fourth embodiment of thepresent invention can also provide similar advantages to those obtainedwith the first embodiment. Note that the driving circuit 79, the pulsegenerator 80, and the sample hold circuit 81 jointly constitutes currentintermittent supply means for the temperature detecting resistors.

The arrangement of principal components of an operation control unitfor, e.g., an automobile, in which the heating resistance flow ratemeasuring apparatus according to any of the first to fourth embodimentsof the present invention is employed, will be described below withreference to FIG. 16.

As shown in FIG. 16, a control unit 62 comprises a heating resistanceflow rate measuring apparatus (gas flow sensor) 59, a pressure senor 60,a temperature sensor 61, a regulator 63 for supplying reference power tothe temperature sensor 61, a multiplexer 64 for switching over outputsof the heating resistance flow rate measuring apparatus 59, the pressuresenor 60 and the temperature sensor 61 from one to another, and an ADconverter 65 converting an analog signal selected by the multiplexer 64into a digital signal.

In the control unit 62, a reference voltage for the AD converter 65 isalso applied from the regulator 63. By setting the sensor outputs of theheating resistance flow rate measuring apparatus 59, the pressure senor60 and the temperature sensor 61 to be changed in proportion to theoutput voltage of the regulator 63 in the control unit 62, therefore,the converted output of the AD converter 65 is also changed inproportion to the output voltage of the regulator 63.

Accordingly, the sensor outputs of the heating resistance flow ratemeasuring apparatus 59, the pressure senor 60 and the temperature sensor61 can be precisely measured regardless of a variation in the outputvoltage of the regulator 63.

1. A heating resistance flow rate measuring apparatus comprising aheating resistor generating heat when a current is supplied to flowtherethrough, and temperature detecting resistors disposed respectivelyupstream and downstream of said heating resistor in the flowingdirection of a fluid, wherein heating suppressing means for suppressingheating of said temperature detecting resistors is disposed to make anoutput voltage of said heating resistance flow rate measuring apparatussubstantially proportional to a voltage value of power source suppliedto said heating resistance flow rate measuring apparatus.
 2. A heatingresistance flow rate measuring apparatus comprising a heating resistorgenerating heat when a current is supplied to flow therethrough, andtemperature detecting resistors disposed respectively upstream anddownstream of said heating resistor in the flowing direction of a fluid,said heating resistance flow rate measuring apparatus being suppliedwith power source from control means for controlling a control target inaccordance with an output signal of said heating resistance flow ratemeasuring apparatus, wherein heating suppressing means for suppressingheating of said temperature detecting resistors is disposed to make anoutput voltage of said heating resistance flow rate measuring apparatussubstantially proportional to a voltage value of the power sourcesupplied from said control unit.
 3. A heating resistance flow ratemeasuring apparatus comprising a heating resistor generating heat when acurrent is supplied to flow therethrough, and temperature detectingresistors disposed respectively upstream and downstream of said heatingresistor in the flowing direction of a fluid, said heating resistanceflow rate measuring apparatus being supplied with power source at alimited voltage and current, wherein heating suppressing means forsuppressing heating of said heating resistor is disposed to make anoutput voltage of said heating resistance flow rate measuring apparatussubstantially proportional to a voltage value of the power sourcesupplied to said heating resistance flow rate measuring apparatus.
 4. Aheating resistance flow rate measuring apparatus according to claim 1,wherein said heating suppressing means is to set a resistance value ofsaid temperature detecting resistors to fall in the range of 5 kΩ to 500kΩ.
 5. A heating resistance flow rate measuring apparatus according toclaim 4, wherein the resistance temperature coefficient of said heatingresistor is in the range of 2000 ppm/° C. to 1000 ppm/° C.
 6. A heatingresistance flow rate measuring apparatus according to claim 4, wherein atotal of a resistance value of said heating resistor and a resistancevalue of wired lines for said heating resistor is set to be within ±20%of a value resulting from dividing a minimum operation assurance voltageof the power source supplied to said heating resistance flow ratemeasuring apparatus by a maximum allowable current of the power sourcesupplied to said heating resistance flow rate measuring apparatus.
 7. Aheating resistance flow rate measuring apparatus according to claim 4,wherein said heating resistor is made of polysilicon.
 8. A heatingresistance flow rate measuring apparatus according to claim 4, whereinsaid temperature detecting resistors are made of polysilicon.
 9. Aheating resistance flow rate measuring apparatus according to claim 4,wherein an area of said heating resistor is about 0.1 mm².
 10. A heatingresistance flow rate measuring apparatus according to claim 4, whereinsaid heating resistor is formed of a plurality of strip-shapedresistors.
 11. A heating resistance flow rate measuring apparatusaccording to claim 4, wherein said heating resistance flow ratemeasuring apparatus includes a resistance connected in series to saidheating resistor, and said resistance has a larger resistance value thansaid heating resistor.
 12. A heating resistance flow rate measuringapparatus according to claim 1, wherein said heating suppressing meansis a Peltier device disposed near each of said temperature detectingresistors and having a temperature cooling function.
 13. A heatingresistance flow rate measuring apparatus according to claim 1, whereinsaid heating suppressing means is means for intermittently supplying acurrent to said temperature detecting resistors.